Selectivity in a xenon difluoride etch process

ABSTRACT

A method and an apparatus for etching microstructures and the like that provides improved selectivity to surrounding materials when etching silicon using xenon difluoride (XeF2). Etch selectivity is greatly enhanced with the addition of hydrogen to the process chamber.

FIELD OF THE INVENTION

The present invention relates to a method of etching which providesimproved selectivity to surrounding materials when etching silicon usingxenon difluoride (XeF₂). In particular, the etch selectivity to siliconnitride is greatly enhanced with the addition of another gas.

BACKGROUND OF THE INVENTION

In the manufacture of microstructures, for examplemicroelectro-mechanical structures (MEMS), gas-phase etching processesare used to remove sacrificial (i.e. unwanted) areas of material so asto leave behind the remaining material which constitutes the desiredstructure.

For example, xenon difluoride (XeF₂) is commonly used to removesacrificial areas of silicon in the manufacture of MEMS. XeF₂demonstrates high selectivity and has a relatively high etch rate whenetching silicon. However, for the manufacture of more complex and higherquality MEMS devices, it is desirable to improve the selectivity of XeF₂processes over conventional techniques.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda method of etching silicon (Si) in a process chamber to produce one ormore microstructures, the method comprising the steps of:

-   -   (a) producing an etching material vapour comprising xenon        difluoride (XeF₂) from an etching material source;    -   (b) transporting etching material vapour to the process chamber;        and    -   (c) introducing a secondary gas comprising hydrogen to the        process chamber.

XeF₂ gas etches Si with the primary reaction as defined by the followingexpression:

2XeF2+Si→2Xe+SiF4  (1)

This reaction is well known, however the Applicant has discovered thatthe use of hydrogen as a secondary gas results in a highly significantincrease in the quality and selectivity of etch that may be achieved.

Preferably, the step of transporting the etching material vapour to theprocess chamber comprises supplying a carrier gas to the etchingmaterial source, the carrier gas thereafter carrying the etchingmaterial vapour to the process chamber.

Alternatively, or additionally, the step of transporting the etchingmaterial vapour to the process chamber comprises employing one or moreexpansion chambers to collect etching material vapour from the etchingmaterial source.

Preferably, the method comprises the additional step of controlling theamount of etching material vapour within the process chamber bycontrolling a vacuum pumping rate out of the process chamber.

Alternatively, the method comprises the additional step of circulatingthe etching material vapour.

Preferably, the method comprises the additional step of providing a maskoverlying the silicon so as to allow selective etching of the silicon.

According to a second aspect of the present invention, there is provideda gas phase etching apparatus for etching silicon (Si) to produce one ormore microstructures, the apparatus comprising:

-   -   a process chamber for receiving silicon to be etched;    -   a xenon difluoride vapour source;    -   a first gas line connecting the xenon difluoride vapour source        to the process chamber;    -   a hydrogen gas source; and    -   a second gas line connecting the hydrogen gas source to the        process chamber.

Preferably, the apparatus further comprises a carrier gas source tocarry xenon difluoride vapour from the xenon difluoride vapour source tothe process chamber.

Alternatively, or additionally, the apparatus further comprises one ormore expansion chambers to collect etching material vapour from theetching material source.

Further alternatively, the second gas line is connected to the xenondifluoride vapour source, the hydrogen gas source employed to carryxenon difluoride vapour to the process chamber.

Preferably, the apparatus further comprises a vacuum pump connected tothe process chamber, the amount of etching material vapour and/orhydrogen gas within the process chamber being controlled by controllingthe pumping rate of the vacuum pump.

Alternatively, or additionally, the apparatus further comprises one ormore flow controllers connected to the first and or the second gas linesto control the amount of etching material vapour and/or hydrogen gaswithin the process chamber.

Alternatively, the apparatus is configured so as to circulate theetching material vapour and/or hydrogen gas.

BRIEF DESCRIPTION OF THE FIGURES

The present invention will now be described by way of example only andwith reference to the accompanying figures in which:

FIG. 1 illustrates in schematic form a gas delivery setup for an etchprocess in accordance with the present invention;

FIG. 2 illustrates in schematic form a layer of PECVD Silicon Nitride ontop of a silicon wafer (a) before etching and (b) after etching;

FIG. 3 is a photograph illustrating the improved selectivity achievedusing an etch process in accordance with the present invention; and

FIG. 4 presents enlargements of the upper left regions of the wafersshown in FIG. 3.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, there is presented a gas delivery system 1that enables an improved selectivity etch process to be carried out, aswill be described in detail below.

The system comprises a carrier gas source 3 which provides a carriergas, the flow rate of which is determined by a mass flow controller(MFC) 5, and a sublimation chamber 7 in which a xenon difluoride (XeF₂)source 9 sublimates to produce an etchant vapour to be carried to theprocess chamber 11 by the carrier gas 3. The carrier gas 3 is preferablyan inert gas such as helium, or alternatively may comprise nitrogen or anitrogen-based gas. The sublimation chamber 7 has an inlet 13 (for thecarrier gas) above the XeF₂ source 9 (crystals) and an outlet 15 (forthe carrier gas plus etchant vapour) beneath the XeF₂ source 9 whichimproves the take-up of etchant vapour by the carrier gas 3. Of course,the inlet 13 and outlet 15 may be arranged in the opposite sense.

The outlet 15 is connected to the process chamber by a supply line 17. Apressure gauge 19 monitors the pressure in the process chamber 11. Thepumping rate of the vacuum pump 21 and/or the MFC 5 can be controlled tomaintain a set operating pressure within the process chamber 11. The useof an adaptive pressure controller 23 (APC) enables accurate control ofthe process chamber pressure. Note that in an alternative embodiment,the gases within the process chamber may be circulated, in which casethe vacuum pump serves to initially evacuate the process chamber (thusdrawing in etchant vapour) or evacuate the process chamber subsequent tocompletion of the etch step.

Also connected to the supply line 17 (or, alternatively, directly to theprocess chamber) is an additional gas line 25 connected to an additionalor secondary gas source 27. Similarly to the carrier gas line 17, theflow rate of the additional or secondary gas is determined by a massflow controller (MFC) 29. Accordingly, the amount of the additional orsecondary gas 27 flowing into the process chamber 11 along with thecarrier gas 3 and etchant vapour can be controlled.

U.S. Pat. No. 6,290,864 in the name of Patel et al, teaches improvementof etch selectivity by noble gas or halogen fluorides by the addition ofnon-etchant gaseous additives having a molar averaged formula weightbelow that of molecular nitrogen. The preferred additive gas is helium,neon, or mixtures of helium and/or neon with one or more of higherformula weight (for example, nitrogen and argon) although it issuggested that any non-etchant gas may be used. Particularly preferredare helium and mixtures of helium and nitrogen or argon.

Patel et al presents experimental results to substantiate the claimedselectivity improvement achieved using the preferred additives. Forexample, a selectivity improvement of 5× is achieved using one of N₂, Arand He.

However, the Applicant in the case of the present Application has made asurprising discovery that the use of hydrogen as an additional gasprovides selectivity improvements on the order of hundreds. By way ofexample, FIG. 2 illustrates in schematic form a layer of PECVD siliconnitride 31 on top of a silicon wafer 33 (a) before etching and (b) afteretching. The nitride layer 31 was patterned and used as a mask to etchthe underlying silicon substrate 33 using XeF_(2.)

To quantify the improved etch selectivity, recipes producing the sameetch rate were used to give a fair comparison between the standardrecipe and the improved selectivity recipe. For example, a carrier gasof N₂ flowing at 50 sccm will transport 25 sccm of XeF₂ to the etchchamber, with the chamber set at 9 Torr. The improved recipe has inaddition a H₂ flow of 20 sccm. A two minute etch was performed, at whichtime the silicon undercut etch 35 measured 6 μm. The following tableprovides a summary of the comparison between the after-etch thicknessesof the nitride mask 31 using the standard and the improved recipe:

Nitride thickness (Å) Standard recipe Before 3212 After 644 Nitrideremoved 2568 Improved recipe Before 3192 After 3183 Nitride removed 9

(The nitride thickness was measured at a number of different locationsso as to provide the above mean values).

As is readily apparent from the tabulated numbers, the selectivity ofthe improved selectivity recipe is, on average, approximately 270× thatof the standard recipe. The improvement achieved is also apparent onvisual inspection. FIG. 3 is a photograph of the test wafers (withenlargements of the upper left regions presented in FIG. 4) from theabove comparison. It is clear that the selectivity on the second testwafer 43 (improved selectivity recipe) is a vast improvement over theselectivity on the first test wafer 41 (standard selectivity recipe).

It is also noted that in Patel et al, the selection of non-etchantgaseous additives having a molar averaged formula weight below that ofmolecular nitrogen is a somewhat arbitrary selection based on the effecton the etch time, and not because of any demonstrated reduction inselectivity above this value. Importantly, no link between said molaraveraged formula weight of the gaseous additive and the improvement inselectivity has been presented—although it is also noted that higherformula weight non-etchant gases are preferred.

A number of alternative embodiments of the present invention areenvisaged (but not necessarily illustrated in the drawings). Forexample, the hydrogen gas may be employed as the carrier gas.Alternatively, and rather than employing a carrier gas to transport theetch vapour to the process chamber, one or more expansion chambers maybe employed in which reserves of vapour are collected and pumped ortransferred to the process chamber as and when required. A carrier gasmay of course be used in conjunction with one or more expansionchambers.

While the described embodiment employs a vacuum pump which pumps carriergas, etch vapour, etch by-product and the secondary hydrogen out of theprocess chamber, thus creating a flow of etchant and hydrogentherethrough, it is also envisaged that the etchant and hydrogen couldbe recirculated.

Further investigation of the mechanisms leading to the greatly enhancedselectivity have been undertaken in an attempt to understand the processinvolved. In Patel et al, it is seen that there is an improvement inselectivity by adding a buffer gas. There appears to be very little ifany improvement in selectivity between buffer gases used. The additionof H₂ in accordance with the present invention provides a dramaticimprovement and is believed to be due to the different reason for theadditive.

A blanket silicon nitride wafer, no silicon etch taking place, is notetched by XeF₂. However, it is observed that when etching silicon withXeF₂ the surrounding nitride is etched. This suggests that a by-productof the etch is reacting with the silicon nitride. The hitherto unknownstep of introducing the additional H₂ gas is believed to create areaction with the by-products before they can react with the siliconnitride. This is not the mechanism described in Patel et al.

-   -   The applicant has made the surprising discovery that adding H₂        to the process chamber improves the silicon etch selectivity to        oxide and nitride. The H₂ is understood to react with etch        by-products that etch the oxide and nitride. These etch        by-products also etch silicon, so a result of adding H₂ to the        process is that the silicon etch rate drops. This drop is        between 10-50% depending on the structure being etched and the        process being used.

(Note that the etch by-product can be a result of an incomplete etchreaction, and as such may be SiF, SiF₂, etc. Also, as the etch isexothermic the heat generated can cause XeF₂ to break up producing Fthat will also react with the silicon, silicon dioxide and siliconnitride.)

Further modifications and improvements may be added without departingfrom the scope of the invention herein described. For example, while theinvention is illustrated using an example wherein a carrier gas is usedto transport etch vapour from the sublimation chamber to the processchamber, it is foreseen that the carrier line could comprise a singleconduit or, as described above, comprise one or more expansion chambersor the like instead.

1. A method of etching silicon (Si) in a process chamber to produce oneor more microstructures, the method comprising the steps of: (a)producing an etching material vapour comprising xenon difluoride (XeF₂)from an etching material source; (b) transporting etching materialvapour to the process chamber; and (c) introducing a secondary gascomprising hydrogen to the process chamber.
 2. The method according toclaim 1, wherein the step of transporting the etching material vapour tothe process chamber comprises supplying a carrier gas to the etchingmaterial source, the carrier gas thereafter carrying the etchingmaterial vapour to the process chamber.
 3. The method according to claim1, wherein the step of transporting the etching material vapour to theprocess chamber comprises employing one or more expansion chambers tocollect etching material vapour from the etching material source.
 4. Themethod according to claim 1, comprising the additional step ofcontrolling the amount of etching material vapour within the processchamber by controlling a vacuum pumping rate out of the process chamber.5. The method according to claim 1, comprising the additional step ofcirculating the etching material vapour.
 6. The method according toclaim 1, comprising the additional step of providing a mask overlyingthe silicon so as to allow selective etching of the silicon.
 7. A gasphase etching apparatus for etching silicon (Si) to produce one or moremicrostructures, the apparatus comprising: a process chamber forreceiving silicon to be etched; a xenon difluoride vapour source; afirst gas line connecting the xenon difluoride vapour source to theprocess chamber; a hydrogen gas source; and a second gas line connectingthe hydrogen gas source to the process chamber.
 8. The apparatusaccording to claim 7, further comprising a carrier gas source to carryxenon difluoride vapour from the xenon difluoride vapour source to theprocess chamber.
 9. The apparatus according to claim 7, furthercomprising one or more expansion chambers to collect etching materialvapour from the etching material source.
 10. The apparatus according toclaim 7, wherein the second gas line is connected to the xenondifluoride vapour source, the hydrogen gas source employed to carryxenon difluoride vapour to the process chamber.
 11. The apparatusaccording to claim 7, further comprising a vacuum pump connected to theprocess chamber, the amount of etching material vapour and/or hydrogengas within the process chamber being controlled by controlling thepumping rate of the vacuum pump.
 12. The apparatus according to claim 7,further comprising one or more flow controllers connected to the firstand or the second gas lines to control the amount of etching materialvapour and/or hydrogen gas within the process chamber.
 13. The apparatusaccording to claim 7, wherein the apparatus is configured so as tocirculate the etching material vapour and/or hydrogen gas.